Method of coating metals with a boride



United States Patent Office 3,251,719 Patented May 17, 1966 3,251,719 METHOD OF COATING METALS WITH A BORIDE Frederick Tepper, Butler, and John Wilson Mausteller and John C. Gerken, Forward Township, Butler County, Pa., assignors to MS.A. Research Corp., Pittsburgh, Pa., a corporation of Pennsylvania -No Drawing. Filed June 19, 1962, Ser. No. 203,462 a 23 Claims. (Cl. 148-611) This invention relates to a method of treating metals and more particularly to coating base metals with a 'Another object is to provide such a method which is simple, direct and easily performed. These and other objects will be apparent from the following description.

Broadly, this invention accomplishes the formation of a boride coating on base metals by contacting the base metal at an elevated temperature with a molten alkali metal bath which contains boron or a boron compound, such as a boron oxide or a boron halide.

It has been found that the reaction for the production of continuous boride coatings in accordance with the present invention preferably are conducted at about 1600 F. to 1800" F. or higher, but can be conducted at temperatures as low as about 1300 F. It is generally desired to maintain the physical properties of the base metals and, in that case, the temperature used should be below that at which the base metal will be substantially altered.

The base metals suitable for use in this invention are the Group IVA metals, namely, titanium, zirconium and hafnium; the Group VA metals, namely vanadium, columbium and tantalum; the Group 111A metals, including the rare earths and actinides, that melt above the temperature required for coating, such as scandium, yttrium, lanthanum, praseodymium, uranium; manganese, iron, nickel and tungsten; and also alloys having a major proportion of these suitable metals, such as, for example, stainless steels, Inconel and Hastalloy. Most of these base metals having melting points well above the temperature required for coating and therefore may be easily coated without adversely affecting the physical properties of the base l (Na B O areespecially'attractive because of their low cost. When using a boron oxide boron source for coating actinides, Group IVA, and Group VA metals, which form stable oxides not reducible by an alkali metal, the boride coating formed may contain some base metal oxide. The various boron halides, e.g. boron trifluoride, boron trichloride, boron tribromide, etc., and boron hydrides, such as dibor-ane, pentaborane, 'decaborane, etc. also are suitable. In many cases the by-product alkali metal salt, formed when using a boron source other than elemental boron, such as sodium halide, hydroxide or sulfide, remains at least partially dissolved in the alkali metal bath. We have found that the use of the boron compounds and/or the presence of the by product salts in the bath does not generally adversely affect the functional utility of the coating, but that a longer reaction time or higher temperature is frequently required to obtain a coating thickness comparable to that obtained when using elemental boron.

All of the alkali metals and mixtures thereof are suit able for use, atlhough it is preferred to use sodium, potassium or their alloys since they are the least expensive. When using a boron oxide boron source it is preferred to use an alkali metal bath containing lithium, sodium or potassi-um because the mate of coating formation is very slow if only rubidium or cesium is used. The alkali metal bath is pyrophoric and must, therefore, be protected from the atmosphere, most conveniently by maintaining an atmosphere of argon or other inert gas over the bath. Commercial alkali metals generally contain some oxide, e.g.,

' commercial sodium may contain 0.1% sodium oxide,

much of which can be removed by filtration of the melted metal at comparatively low temperature. When using elemental boron as a boron source it is generally preferred to use filtered, or otherwise deoxidized, alkali metal to realize the full advantage of an oxide-free bath.

The method is simple to conduct since it comprises merely placing the base metal in the alkali metal bath which contains a boron source material at the aforementioned elevated temperature. The proportions of boron source material and alkali metal in the bath are not critical, so long as there is sufficient alkali metal to provide a fluid media for good contact with the base metal; generally it is preferred to use a major proportion of alkali metal.

Conveniently, a solid boron source material and the base metal to be coated are placed in a suitable container, such as stainless steel or carbon steel, having a closure with inlet and outlet connections for inert gas and alkali metal. The container is then closed and flushed with inert gas to remove the enclosed air; sufiicicnlt molten alkali metal is then introduced to the container to cover the base metal and the bath is heated to and maintained at the desired temperature for the desired length of time. The bath can then be drained from the container and the coated metal piece removed. Fluid boron sources, such as boron halides and boron hydrides, are conveniently bubbled or injected into a molten alkali metal bath. If desired, the bath of alkali metal and boron source material may be preformed and the base metal introduced and withdrawn from the bath through inert gas air locks V which prevent atmosphere contamination. The boron source material can be added to the bath periodically or continuously to replenish that consumed in forming the coating.

The coatings formed by this invention are borides of the base metal, and are believed to result from reaction of the base metal surface with the alkali metal bath containing a boron source material. Coatings of appreciable thickness are apparently formed by diffusion of boron or borides into the base metal. The coatings vary in thickness depending on the conditions of operation, particularly including the type of base metal employed, the time and temperature of operation and, to some extent, the amount of boron source material in the alkali metal bath. Continuous coatings from about 0.0005 inch to 0.005 inch in thickness have been obtanied.

The following examples are illustrative of this invention and are not intended in any sense to limit the manner in which this inveniton can be practiced.

Example I Twenty-five grams of boric oxide and a titanium rod 1% inches in diameter and 1 inch long, supported on a stainless steel screen, were placed in a closed stainless steel container with suitable inlet and outlet connections. The container was flushed with argon and 125 g. of liquid sodium was introduced, which completely immersed the titanium rod. The container was placed in a tfurnace and heated to 1800 F. for six hours, cooled to just about the melting point of sodium, and the melt was drained from the container. The titanium rod, washed in alcohol to remove residual sodium, was coated with a continuous coating of titanium diboride, identified by X-ray diffraction analysis, which added 0.030 g. to the sample.

Example II Example I was repeated except that 4.0 g. of boron and 13 g. of boric oxide were used in place of the'boric oxide and substantially the same results were obtained.

Example. III

Example I was repeated except that 9.0 g. of boronwas used instead of the boric oxide and the temperature was 1600 F. The titanium sample was coated with a continuous titanium boride coating 0.0024 inch thick after six hours, and 0.0044 inch thick after twenty-four hours.

Example IV The reactor used in Example I was equipped with stirrer. Example I was repeated using filtered sodium,

a temperature of 1600 F., 9.0 g. of boron instead of the boric oxide, and a section of nickel pipe in place of the.

titanium rod. After a six hour treatment the pipe was coated on all surfaces with a continuous nickel boride coating 0.0046 inch thick, and after a twenty-four hour treatment the coating thickness was 0.0083 inch.

Example VI Example V was repeated using a section of type 304 stainless steel pipe in place of the nickel pipe. After six hours treatment the pipe was coated on all surfaces with a 0.0015 inch thick bon'de coating.

Example VII Example V was repeated .using Inconel pipe in place of the nickel pipe. After 6 hours treatment the boride coating formed on the pipe was 0.0016 inch thick, and

after 12 hours treatment it was 0.0023 inch thick.

Example VIIl Example V was repeated using Hastalloy B tubing in place of the nickel pipe and a .OOIS-inch boride coating was formed on all surfaces of the tube.

Example 1X Example V is repeated a number of times using respectively in place of the nickel pipe, a piece of zirconium, hafnium, vanadium, columbium, tantalum, scandium, yttrium, praseodymium, thon'um, uranium, iron, manganese and carbon steel, and in each case a boride coating is formed on the metal piece similarly as in Example V.

From the foregoing description and examples, it is seen that continuous boride coatings are formed on base metals by the direct and simple method of this invention.

According to the provisions of the patent statutes, we have explained the principle of our invention and have illustrated and described what we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

4 We claim: 1. A method of forming a boride coating on a base metal comprising contacting a base metal selected from the group consisting of the periodic groups IVA, VA, IIIA ineluding the rare earth and actinide elements, manganese, iron, nickel, tungsten and alloys thereof, with a bath consisting of a major portion of alkali metal and a minor proportion of a boron component selected from the group consisting of elemental boron and the halides, oxides, mixed metal oxide and hydtides of boron, under a nonoxidizing atmosphere at a temperature of at least about 2. A method of claim 1 in which the temperature is between about 1600 F. and 18009 F.

3. A method of claim 2 in which the boron component is boron oxide and the alkali metal is selected from the group consisting of lithium, sodium, potassium and mixtures thereof.

4. A method of claim 3 in which the base metal is nickel.

5. A method of claim 3 in which the base metal is stainless steel.

6. A method of claim 3 in which .the base metal is an alloy containing a major proportion of nickel.

7. A method of claim 2 in which the boron component is elemental boron.

8. A method of claim 7 in which the alkali metal is substantially free of oxide.-

9. A method of claim 8in which the base metal is titanium.

10. A method of claim 8 in which the base is hafnium.

11. A method of claim 8 in which the base metal is vanadium. A

12. A method of claim 8 in which the base columbiurn.

13. A method of claim 8 in tantalum.

14. A method of claim 8 in scandium.

15.A method of claim 8 in yttrium.

16. A method of claim 8 in thorium.

17.A method of claim 8 in metal is which the base metal is which the base metal is which the base metalis which the base which the base metal is uranium.

which the base which the base metal is which the base which the base metal is which the base metal is References Cited by the Examiner UNITED STATES PATENTS 1,855,562 4/1932 Swinne 148-635 X 2,823,151 2/1958 Yntema et al 14 86.3 X, 2,848,352 8/1958 Noland et al 117--1l4 X 2,854,353 9/1958 Schwope. 2,929,740 3/1960 Logan 1l7ll4 3,085,028 4/1963 Logan 117-114 RICHARD D. NEVIUS, Primary Examiner. R. S. KENDALL, Assistant Examiner.

metal is metal is metal is 

1. A METHOD OF FORMING A BORIDE COATING ON A BASE METAL COMPRISING CONTACTING A BASE METAL SELECTED FROM THE GROUP CONSISTING OF THE PERIODIC GROUPS IVA, VA, IIIA INCLUDING THE RARE EARTH AND ACTINIDE ELEMENTS, MANGANESE, IRON, NICKEL, TUNGSTEN AND ALLOYS THEREOF, WITH A BATH CONSISTING OF A MAJOR PORTION OF ALKALI METAL AND A MINOR PROPORTION OF A BORON COMPONENT SELECTED FROM THE GROUP CONSISTING OF ELEMENTALBORON AND THE HALIDES, OXIDES, MIXED METAL OXIDE AND HYDRIDES OF BORON, UNDER A NONOXIDIZING ATMOSPHERE AT A TEMPERATURE OF AT LEAST ABOUT 1300*F. 